1. Field of the Invention
The present invention relates generally to a control circuit for operating a power supply circuit having a SCR starter, and more particularly to an improved control circuit for supplying operating current to a motor through a bypass circuit and for effectively disconnecting the SCR starter assembly after the motor has reached the operating speed.
2. Description of the Prior Art
When a motor is initially connected to a power source, the in-rush current supplied to the motor may exceed the nameplate current by a factor of from five to seven times. This has the possible delitescent effect of either exceeding the power supply capability, or interfering with other users of such power supply, or both.
In order to limit this in-rush current, it is known to provide an SCR starter between the power source and the motor. However, after the motor has reached its operating speed, it is also desirable to effectively remove the SCR starter from the power supply circuit to prevent unnecessary wear on the starter assembly and to prevent possible imbalance in power supplied to the operating motor. To accomplish this, it is further known to provide a bypass circuit around the SCR starter. Hence, after the motor has reached its operating speed, operating current may thereafter be supplied through the bypass circuit, and the SCR starter may be thereafter disconnected or deactivated.
One example of such known circuit is shown in Canadian Patent No. 796,414, which provides for simultaneous completion of the bypass circuit and deactivation of the SCR starter.
However, such simultaneous switching has a number of significant disadvantages. As a practical matter, it is virtually impossible to effect simultaneous switching. If the SCR starter is disconnected before the bypass circuit is completed, the subsequent closure of the bypass circuit may again cause an in-rush current. Moreover, arcing may occur if the SCR starter is deactivated before the SCR bypass circuit is completed.